Processed semiconductor dice containing integrated circuits may be electrically and physically connected with other semiconductor dice, semiconductor dice in wafer or partial wafer form, interposers, circuit boards, and other higher-level packaging, any such structures hereinafter collectively referred to as “substrates,” to operably connect the integrated circuits on the semiconductor die with those on another substrate. This connection may use a large number of electrically conductive elements protruding from a major surface, such as the active surface or a backside surface, of the semiconductor die or semiconductor dice in wafer or partial wafer form. The conductive elements may comprise an electrically conductive bump, stud, column, or pillar, which may in some instances be in the form of a cylindrical structure. The conductive element may reside on a conductive pad, referred to in the art as a “bond pad,” on an active surface of the semiconductor substrate.
To accomplish the electrical and physical interconnection, the semiconductor die, which may be included in a wafer or partial wafer form, may be inverted, i.e., flipped upside down, and bonded to another conductive material, referred to in the art as a “landing pad,” on another substrate. Bonding, which may be effected using a solder material that is melted and then solidified, is accomplished during a processing stage known as “die attach.” Thus, during die attach, the semiconductor die, which may be referred to in the art as “die one,” is electrically and physically interconnected with the substrate, which, if the substrate comprises the base semiconductor die of a stacked die assembly, may be referred to as “die zero.” Multiple such dice may be stacked upon one another in this manner to form a stacked semiconductor die assembly. If the semiconductor die was in a wafer form during die attach, the wafer may be singulated to form individual or groups of processed semiconductor dice.
As noted above, a solder material may be used to accomplish the die attach. The solder material may be in the form of solder mass, also known in the art as a “solder ball” or a “solder bump,” supported by another material of the conductive element or directly by a bond pad. During die attach, the solder may be reflowed in proximity with the conductive element and the landing pad, optionally in the presence of a flux material.
Before or during reflow of the solder, during die attach, during die stacking, during subsequent processing, or any combination thereof, the solder may slump, or even wick, along the periphery of a supporting material of the electrically conductive element. Either or both of solder slumping or wicking may lead to undesirable formation of intermetallic materials due to reaction between one or more materials within the solder and one or more materials within the supporting material of the conductive element. For example, conventional conductive elements may include a conductive material, e.g., copper, which may react and form an intermetallic material with a material of a conventional solder material used for die attach, e.g., tin, disposed over the conductive material, for example, at an end of the conductive element. Growth of such a copper-tin intermetallic material may proceed at a significant pace, leading to deterioration of the conductive pillar and potential formation of Kirkendall voids within the conductive element. The formation of the intermetallic material can cause problems during temperature cycling testing and high-temperature storage testing as well as during operation of the semiconductor dice employing the conductive elements.
Solder wicking or slumping can also leave an insufficient volume of solder disposed between the conductive element and the landing pad, weakening the strength of the bond. Substantial solder wicking or slumping can lead to failure of the joint between the conductive element and landing pad.
Conventional conductive elements may include a conductive barrier material disposed between the conductive material of the conductive element and the region of the conductive element in contact with the solder material during solder reflow and die attach. The barrier material may, at least initially, prevent direct contact between the conductive material and the solder. However, the presence of the barrier material does not necessarily inhibit the solder from slumping or wicking along the sides of the electrically conductive element and undesirably coming into contact with the conductive material of the conductive element. Therefore, use of a barrier material on the conductive element may not, by itself, sufficiently inhibit solder wicking and slumping, formation of intermetallic materials, bond weakness, or joint failure.